Objection 1. It would seem that the children who died in original sin were delivered from hell by Christ's descending thither. For, like the holy Fathers, the children were kept in hell simply because of original sin. But the holy Fathers were delivered from hell, as stated above (Article 5). Therefore the children were similarly delivered from hell by Christ.
Objection 2. Further, the Apostle says (Romans 5:15): "If by the offense of one, many died; much more the grace of God and the gift, by the grace of one man, Jesus Christ, hath abounded unto many." But the children who die with none but original sin are detained in hell owing to their first parent's sin. Therefore, much more were they delivered from hell through the grace of Christ.
Objection 3. Further, as Baptism works in virtue of Christ's Passion, so also does Christ's descent into hell, as is clear from what has been said (4, ad 2, 5,6). But through Baptism children are delivered from original sin and hell. Therefore, they were similarly delivered by Christ's descent into hell.
On the contrary, The Apostle says (Romans 3:25): "God hath proposed Christ to be a propitiation, through faith in His blood." But the children who had died with only original sin were in no wise sharers of faith in Christ. Therefore, they did not receive the fruits of Christ's propitiation, so as to be delivered by Him from hell.
I answer that, As stated above (Article 6), Christ's descent into hell had its effect of deliverance on them only who through faith and charity were united to Christ's Passion, in virtue whereof Christ's descent into hell was one of deliverance. But the children who had died in original sin were in no way united to Christ's Passion by faith and love: for, not having the use of free will, they could have no faith of their own; nor were they cleansed from original sin either by their parents' faith or by any sacrament of faith. Consequently, Christ's descent into hell did not deliver the children from thence. And furthermore, the holy Fathers were delivered from hell by being admitted to the glory of the vision of God, to which no one can come except through grace; according to Romans 6:23: "The grace of God is life everlasting." Therefore, since children dying in original sin had no grace, they were not delivered from hell.
Reply to Objection 1. The holy Fathers, although still held bound by the debt of original sin, in so far as it touches human nature, were nevertheless delivered from all stain of sin by faith in Christ: consequently, they were capable of that deliverance which Christ brought by descending into hell. But the same cannot be said of the children, as is evident from what was said above.
Reply to Objection 2. When the Apostle says that the grace of God "hath abounded unto many," the word "many" [The Vulgate reads 'plures,' i.e. 'many more'] is to be taken, not comparatively, as if more were saved by Christ's grace than lost by Adam's sin: but absolutely, as if he said that the grace of the one Christ abounded unto many, just as Adam's sin was contracted by many. But as Adam's sin was contracted by those only who descended seminally from him according to the flesh, so Christ's grace reached those only who became His members by spiritual regeneration: which does not apply to children dying in original sin.
Reply to Objection 3. Baptism is applied to men in this life, in which man's state can be changed from sin into grace: but Christ's descent into hell was vouchsafed to the souls after this life when they are no longer capable of the said change. And consequently by baptism children are delivered from original sin and from hell, but not by Christ's descent into hell.
Source: Summa Theologica (New Advent)